test: fix box-luatest/gh-6786_func_index_iterator_stable_test

The is_multikey option must be wrapped in opts, because commit
66f5368b ("schema: allow to pass
is_multikey func opt without sub-table") wasn't backported.

test/box-luatest/gh-6786_func_index_iterator_stable_test.lua

@@ -23,7 +23,7 @@ g.before_test('test_func_index_iterator_stable', function()
             end]],
             is_deterministic = true,
             is_sandboxed = true,
-            is_multikey = true,
+            opts = { is_multikey = true },
         })
         local s = box.schema.create_space('test')
         s:create_index('pk')